This invention generally relates to flow control mechanisms and more specifically to a fluid flow control apparatus preferably for use in space to control the flow of propellant to an electrothermal thruster.
Electrothermal thrusters, such as arcjets or resistojets, are utilized in space applications for attitude control and station keeping of satellites. These thrusters generally use a gaseous or vapor propellant feed. Storage of propellants as liquids rather than as gases is more attractive for technical and economic reasons. "Storable" liquid propellants, such as water or ammonia, must therefore be vaporized on orbit prior to use in a cold gas or electric thruster.
Vaporization of a storable liquid propellant for electric propulsion devices in the zero gravity environment of space is complicated by the following:
1. Maintenance of good thermal contact between the liquid and the heating surfaces is not aided by gravity. Suitable heat transfer coefficients are therefore difficult to achieve. PA1 2. Unstable or oscillatory flow can occur due to improper feedstream impedance. This problem is characteristic of the once through boilers (vaporizers) typical of space applications. PA1 3. Variable control of flow rate, i.e. throttling, if desired, is very difficult for the low flow rates typical of electric thrusters used for attitude control and station keeping such as resistojets and arcjets. For typical mass flow rates of less than 1 lbm/hr, liquid volumetric flow rates are so low that control valve resolution is inadequate. Vapor phase volumetric flow rates are higher, to be sure, but hot-gas flow control valves are subject to material compatibility and reliability problems which are not compatible with the high reliability and long life requirements of space applications. PA1 (1) Providing a storable liquid propellant in a storage container; PA1 (2) Providing a flow path for the liquid propellant from the storage container containing the propellant to an electric thruster; PA1 (3) Interposing a porous medium in the flow path which has a low liquid permeability (as defined by Darcy's Law) so as to effectively restrict or limit the flow of the liquid propellant through the medium; and PA1 (4) Controllably heating the medium to controllably vaporize at least a portion of the liquid propellant in the medium creating a back pressure in the medium to thereby control the rate of propellant fluid flow to the electric thruster.